1. Field of the Invention
This invention relates to a variable valve train of an internal combustion engine, capable of changing the drive phase of an intake or exhaust valve and the valve lift amount.
2. Description of the Related Art
In order to perform exhaust emission control for an internal combustion engine, such as an automotive engine, or reduce fuel consumption, as is known, the phase or lift amount of a valve of an intake or exhaust system is changed in accordance with the operating state of the internal combustion engine. As a variable valve train to attain this, there is known a vane-type variable-phase valve train in which the cam phase is continuously changed by hydraulic power.
Also known is a valve train of a cam-switching type in which the drive phase and lift amount of a valve is adjusted to the operating state of the internal combustion engine by switching a plurality of types of cams in accordance with the operating state.
Alternatively, as described in Japanese Patent No. 3245492, there is also known a mechanical continuously-variable valve train in which the drive phase and lift amount of a valve can be changed by using a gear that is driven by a stepping motor, an intermediate lever, a return spring, etc.
In the vane-type variable-phase valve train, however, the lift amount of the valve cannot be changed, although the valve drive phase can be shifted by changing the position of the vane.
In the cam-switching valve train or the mechanical continuously-variable valve train, on the other hand, the lift amount and the phase can be shifted. However, the cam-switching valve train requires use of a plurality of types of cams, so that it includes a lot of components and is complicated in construction. Further, the mechanical continuously-variable valve train separately requires a mechanism for changing the lift amount and a mechanism for shifting the phase, so that its construction is complicated and its size is large.
In the case of a conventional continuously-variable-phase valve train, moreover, the valve-opening start timing of an intake valve is also retarded inevitably if the valve closing time is retarded. Thus, a valve overlap between intake and exhaust is reduced or removed, so that a problem arises, such as reduction of fuel efficiency because of pumping loss.